Memory formation in matter

Memory formation in matter is a theme of broad intellectual relevance; it sits at the interdisciplinary crossroads of physics, biology, chemistry, and computer science. Memory connotes the ability to encode, access, and erase signatures of past history in the state of a system. Once the system has completely relaxed to thermal equilibrium, it is no longer able to recall aspects of its evolution. Memory of initial conditions or previous training protocols will be lost. Thus many forms of memory are intrinsically tied to far-from-equilibrium behavior and to transient response to a perturbation. This general behavior arises in diverse contexts in condensed matter physics and materials: phase change memory, shape memory, echoes, memory effects in glasses, return-point memory in disordered magnets, as well as related contexts in computer science. Yet, as opposed to the situation in biology, there is currently no common categorization and description of the memory behavior that appears to be prevalent throughout condensed-matter systems. Here we focus on material memories. We will describe the basic phenomenology of a few of the known behaviors that can be understood as constituting a memory. We hope that this will be a guide towards developing the unifying conceptual underpinnings for a broad understanding of memory effects that appear in materials

Mechanics of bacterial cellulose hydrogel

Natural polymer-based hydrogels, like bacterial cellulose (BC) hydrogels, gained a growing interest in the past decade mainly thanks to their good biological properties and similar fibrous structure as real human tissues that make them good potential candidate materials for various applications in a biomedical field. BC hydrogels are produced in a process of primary metabolism of some microorganisms. They were intensively studied with regard to their biological aspects, revealing many potential applications such as a direct implant replacement of some real tissues and an excellent scaffold for in-vitro tissue regeneration; still, its mechanical behaviour under application-relevant conditions has not been well documented. [Continues.

Continuum damage evaluation and homogenization process in quasi-fragile materials simulated using a lattice discrete element method

Conhecer, predizer e modificar como estruturas atingem o colapso é um desafio para a engenharia e também uma chave tecnológica no desenvolvimento de estruturas. Entre os materiais utilizados, aqueles que possuem comportamento dúctil, como é o caso de metais, apresentam um processo de dano que é estudado dentro da teoria da plasticidade, permitindo manter a hipótese dos meios contínuos até determinado grau de deterioração. No caso de matérias quasi-frágeis, como é o caso de cerâmicos, alguns tipos de rochas e concreto, a hipótese de modelos utilizados em materiais dúcteis é no mínimo discutível quando o nível de dano está em elevados patamares, havendo neste caso fenômenos particulares, como a localização, a iteração entre clusters de microfissuras e o efeito de escala, entre outros. É de interesse relacionar os resultados obtidos dentro do âmbito da mecânica do contínuo com teorias que preveem um conjunto de descontinuidades que podem crescer e interagir. Notavelmente, métodos alternativos baseados na mecânica do descontínuo tem apresentado resultados promissores. Neste cenário, o domínio é representado por nós vinculados entre si por funções de interação baseados em campos de forças. Estes métodos permitem incorporar naturalmente o dano e/ou a fratura. Na presente dissertação, uma versão do método dos elementos discretos é aplicada primeiramente para simular campos descontínuos que tem solução analítica conhecida dentro da mecânica do contínuo. Os parâmetros convencionalmente empregados na mecânica do contínuo e os conceitos de micromecânica são empregados para permitir comparações entre a solução analítica (mecânica do contínuo) e a extraída numericamente (modelo discreto). O efeito da mudança do número de trincas e de seus respectivos tamanhos é também estudada. Numa segunda aplicação, o modelo discreto é submetido a danos progressivos devido a carregamentos cíclicos proporcionais e não-proporcionais, permitindo avaliar como as propriedades mecânicas se degeneram ao longo do tempo. Por fim, é feito um estudo mostrando o efeito da subdivisão do domínio discreto, observando-se o erro associado ao se realizar este tipo de procedimento. Diversas observações feitas durante o trabalho permitem verificar não só a validade da metodologia, mas também interpretar os resultados obtidos dentro de cada teoria.Knowing, predicting and modifying how the structure reaches the collapse is an engineering challenge and also a technological key for the development of structures. Among the materials, those with ductile behavior, as metals, are evaluated considering the damage process within the plasticity theory framework and, in this case, the hypothesis of a continuum medium is accepted up to a certain degree of deterioration. For quasi-fragile materials, such as ceramics, some types of ground stones and concrete, the hypothesis applied in ductile materials models is, at the very least, questionable when the damage level is high. In this situation, singular phenomena like the localization, interaction between the microcracks clusters, scale effect, among others, can happen. It is of interest to relate continuum mechanics results with theories that allow the material to present a set of interactive and growing discontinuities. Notably, the application of methods based on discontinuous mechanics has presented promising results. In this scenario, the domain is represented by nodes bounded with each other through interacting functions based on field forces. These methods permit to incorporate the damage and/or the fracture naturally. Firstly, in the present dissertation, a version of the discrete element method is applied to simulate discontinuous fields with a known analytical solution in the context of the continuum damage mechanics. The parameters conventionally applied in continuum mechanics and the concepts of micromechanics are incorporated to allow comparisons between the known analytical (continuum mechanics) and the extracted numerical approach (discrete model). The effect caused by the number of cracks and their corresponding sizes is also studied. In the second application, the discrete model is submitted to progressive damage due to proportional and nonproportional cyclic loading, allowing to discuss and evaluate how the material properties degenerate over time. Lastly, a scenario showing the effect of the domain’s subdivision is made to visualize the associated error while performing this type of analysis. Many observations made during this work permit to verify not only the validity of the methodology but also to interpret the obtained results in the frameworks of the continuous damage mechanics and fracture/damage mechanics

Mechanical behaviour of thermally bonded bicomponent fibre nonwovens: experimental analysis and numerical modelling

In contrast to composites and woven fabrics, nonwoven materials have a unique web structure, which is composed of randomly oriented fibres bonded in a pattern by mechanical, thermal or chemical techniques. The type of nonwovens studied in this research is a thermally bonded one with polymer-based bicomponent fibres. Such fibres have a core/sheath structure with outer layer (sheath) having a lower melting temperature than that of the core. In thermal bonding of such fibres, as the hot calender with an engraved pattern contacts the fibrous web, bond points are formed thanks to melting of the sheath material. Molten sheath material acts as an adhesive while core parts of the fibres remain fully intact in the bond points. On the other hand, web regions, which are not in contact with the hot engraved pattern, remain unaffected and form the fibre matrix that acts as a link between bond points. With two distinct regions, namely, bond points and fibre matrix, with different structures, nonwovens exhibit a unique deformation behaviour. This research aims to analyse the complex mechanical behaviour of thermally bonded bicomponent fibre nonwoven materials using a combination of experimental and numerical methods. A novel approach is introduced in the thesis to predict the complex mechanical behaviour of thermally bonded bicomponent fibre nonwovens under various threedimensional time-dependent loading conditions. Development of the approach starts with experimental studies on thermally bonded bicomponent fibre nonwovens to achieve a better understating of their complex deformation characteristics. Mechanical performance of single bicomponent fibres is investigated with tensile and relaxation tests since they are the basic constituents of nonwoven fabrics. The fabric microstructure, which is one of the most important factors affecting its mechanical behaviour, is examined with scanning electron microscopy and X-ray micro computed tomography techniques. At the final part of experimental studies, mechanical response of thermally bonded bicomponent fibre nonwovens is characterised with several mechanical tests. (Continues...)

Active thermography for the investigation of corrosion in steel surfaces

The present work aims at developing an experimental methodology for the analysis of corrosion phenomena of steel surfaces by means of Active Thermography (AT), in reflexion configuration (RC). The peculiarity of this AT approach consists in exciting by means of a laser source the sound surface of the specimens and acquiring the thermal signal on the same surface, instead of the corroded one: the thermal signal is then composed by the reflection of the thermal wave reflected by the corroded surface. This procedure aims at investigating internal corroded surfaces like in vessels, piping, carters etc. Thermal tests were performed in Step Heating and Lock-In conditions, by varying excitation parameters (power, time, number of pulse, ….) to improve the experimental set up. Surface thermal profiles were acquired by an IR thermocamera and means of salt spray testing; at set time intervals the specimens were investigated by means of AT. Each duration corresponded to a surface damage entity and to a variation in the thermal response. Thermal responses of corroded specimens were related to the corresponding corrosion level, referring to a reference specimen without corrosion. The entity of corrosion was also verified by a metallographic optical microscope to measure the thickness variation of the specimens

Morphogenesis and Growth Driven by Selection of Dynamical Properties

Organisms are understood to be complex adaptive systems that evolved to thrive in hostile environments. Though widely studied, the phenomena of organism development and growth, and their relationship to organism dynamics is not well understood. Indeed, the large number of components, their interconnectivity, and complex system interactions all obscure our ability to see, describe, and understand the functioning of biological organisms. Here we take a synthetic and computational approach to the problem, abstracting the organism as a cellular automaton. Such systems are discrete digital models of real-world environments, making them more accessible and easier to study then their physical world counterparts. In such simplified synthetic models, we find that the structure of the cellular network greatly impacts the dynamics of the organism as a whole. In the physical world, for example, the network property wherein some cells depend on phosphorus produces the cyclical boom-bust dynamics of algae on the surface of a pond. Using techniques of synthetic biology and cellular automata, such local properties can be abstractly specified, and the long-term, system-wide, and dynamical consequences of localized assumptions can be carefully explored. This thesis explores the potential impacts of Darwinian selection of dynamical properties on long term cellular differentiation and organism growth. The focus here is on the relationship between organism homogeneity (or heterogeneity) and the dynamical properties of robustness, adaptivity, and chromatic symmetry. This dissertation applies an experimental approach to test the following three hypotheses: (1) cellular differentiation increases the expected robustness in an organism’s dynamics, (2) cellular differentiation leads to more uniform adaptivity as the organism grows, and (3) for organisms with symmetry, growth by segment elongation is more likely than growth by segment reduplication. To explore these hypotheses, we address several obstacles in the experimental study of dynamical systems, including computational time limits and big data